1. Field of the Invention
The present invention relates to a zero-voltage-switching (ZVS) piezoelectric driving circuit, and in particular to a piezoelectric driving circuit of a Zero-Voltage-Switching (ZVS) half-bridge circuit, capable of operating in a wide bandwidth and in a wide range of load.
2. The Prior Arts
Due to its advantages of compact size, thin profile, and free of Electromagnetic Interference (EMI), the piezoelectric transformer has been used extensively in the power driving circuits. However, the piezoelectric transformer still has insurmountable problems. By way of example, in applying a piezoelectric transformer into a bridge switching circuit, usually, an inductor has to be connected between the bridge switching circuit and the piezoelectric transformer, to achieve Zero-Voltage-Switching. However, the inductor itself is thicker than the piezoelectric transformer (namely, having a larger dimension), such that it has to sacrifice the advantages of thin profile of the piezoelectric transformer. In addition, the disposition of inductor in the main circuit could cause additional power loss and electromagnetic interference (EMI).
Moreover, by not including an inductor in a piezoelectric transformer driving circuit, though it can achieve Zero-Voltage-Switching (ZVS) for a specific load range, yet the frequency range applicable is just too narrow, so that the piezoelectric transformer is not able to keep up ZVS operations under the conditions of frequency variation feedback control and large range load variations. In fact, without regard to whether piezoelectric transformer is used in cooperation with an inductor, the frequency range for the driving circuit to achieve ZVS is rather narrow. Furthermore, in actual application, piezoelectric transformer is difficult to achieve ZVS for a light load, when it is far away from resonance frequency. For this reason, the development and application of piezoelectric transformer in power supply is rather limited.
Therefore, presently, the design and performance of the piezoelectric transformer is not quite satisfactory, and it has much room for improvements.